Heat Management

Hot black liquor from a previous cook is stored in the tank farm and, as it is

charged to the digester, passes on its heat energy directly to the next cook. The

remainder of hot black liquor not re-used in hot displacement is available for indirect

heat transfer to white liquor and process water in shell-and-tube heat exchangers.

370 4 Chemical Pulping Processes

Typically excess hot black liquor, which is not needed in hot displacement, heats

white liquor coming from the recausticizing plant. The temperature levels in the

other black liquors are normally not high enough to be economically transferred

to white liquor. Nonetheless, they are sufficiently hot for the generation of considerable

amounts of hot water from warm water. The hot water temperature achievable

from cooling of warm black liquor is 80–90 °C. The cooling of wash filtrate

coming from brownstock washing yields somewhat lower water temperatures,

because the filtrate temperature must be low enough to bring the digester contents

safely beneath the boiling point.

In addition, the temperature of hot white liquor and/or hot black liquor is

adjusted in the tank farm by indirect steam heating. Steam heating in the tank

farm has the advantage of continuous steam consumption as compared to digester

circulation liquor heating, which occurs only during the short period of heating

to cooking temperature.